Cast steel and casting mold

ABSTRACT

By limiting steel compositions with respect to special components and controlling contents of Ti, Zr and S in a steel to a suitable range, fine Ti and Zr sulfides are generated so that a cast steel having excellent internal quality and machinability after casting can be ensured. Then, by casting this cast steel a mold having excellent machinability, whose internal quality as cast is comparable to that of a forged steel product, can be produced. Therefore, the mold of the present invention can be widely applied to use, which severely requires excellent surface properties in products for deep-engraving in which a worked surface reaches the internal portion of the section material or for finishing.

[0001] This application is a continuation of International Patent Application No. PCT/JP02/02729. This PCT application was not in English as published under PCT Article 21(2).

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a cast steel having excellent internal quality and excellent machinability after casting and a cast steel product obtained by casting said cast steel, particularly a mold, and more specifically, suppressing various defects in a cast state, relates to a cast steel having excellent internal quality and machinability equal to a forged steel product and a mold using the cast steel.

[0004] 2. Related Art

[0005] A mold is a tool indispensable in molding or forming various materials such as metal, plastics and the like. Various steel materials are usually used as materials of the molds.

[0006] As a material of a plastic forming mold, for example, a carbon steel containing C: 0.1-0.50 mass %, or a steel material of the carbon steel to which Cr, Mo, Cu, Ni, V and the like were added has been used. Further, as a material of a hot forging mold, a material of JIS SKD 61 and a steel material containing Cr: 1-9 mass %, V: 0.2-1.2 mass %, and Mo: 0.2-2.0 mass % as principal components is used. Further, as a material of a cold forming mold a steel material containing, for example, about 1 mass % C and 3.5 mass % Cr is used.

[0007] To produce these molds a steel material is hot-worked to make a section material and is cut off if necessary. Then the section material is often machined to a predetermined mold shape by users. However, when using the above-mentioned method of producing a mold in which after hot working a steel material machining is performed, it takes many days until after purchasing the section material, molds are completed by machining, and there is a limit in shortening delivery times and the like. Further, since the above-mentioned section materials are obtained by hot working, it is difficult to machine at a near-required shape, so called a near net shape, and many parts of section materials are discharged as chips. Additionally it is required tremendous amounts of time and tool cost to machine.

[0008] On the other hand, in a cast material casting can be performed to obtain a product in a shape having various types of cooling holes and similar to a finished product. However, there are following problems and large constrains in the use of the cast material.

[0009] For example, Japanese Patent Application Laid-open No. Hei. 11-279673 proposes use of Zn cast alloy having a low melting point and easy treatment. That is use of zinc alloy consisting of, by mass %, Mg: 1.5-2.5%, Al: 3-5% and Cu: 2-4% by mass and the remnants of substantially Zn, whose solidification starting temperature is 390° C. or less, whose Vickers hardness is 150 or more and in which the generation of porosity was suppressed.

[0010] Further, Japanese Patent Application Laid-open No. Hei. 10-147840 proposes to produce a mold with cast steel. That is the proposal is to produce a mold of a free cutting cast steel consisting of, by mass %, C: 0.5-1.0%, Si: 0.25-1.5%, Mn: 1.0-1.85%, Cr: 0.6-5.0%, one or two of Mo and W: 0.06-5.0% for (Mo+W/2), and S: 0.10-0.40%, and the remnants substantially Fe, in which sulfide type inclusions were granularly dispersed in the base steel matrix.

[0011] However, since the former is an alloy containing Zn as a principal component, it has excellent machinability, but it has limits in hardness, which is necessary for improving the life of mold. Further, the Zn alloy is remarkably expensive as compared with a steel material.

[0012] Further, since the latter uses the same steel composition as in the conventional case, it can have sufficient hardness. However, when S (sulfur) is positively added to the steel to improve machinability whereby MnS content is increased, casting defects such as porosity, and segregation of S or a crack due to them and the like are promoted because of cast steel. Further, since the cast steel is not subjected to hot working in a later step, porosity is not intimately closed by pressing and the porosity becomes surface defects after cutting or machining.

[0013] Therefore, in conventional technology, the cast material could not be applied to use, which severely requires excellent surface properties in products for deep-engraving in which a worked surface reaches the internal portion of the section material or for finishing.

SUMMARY OF THE INVENTION

[0014] The present invention was made to solve the above-described prior art problems. The objects of the present invention are to provide a cast steel having excellent internal quality and excellent machinability after casting, and to provide a mold obtained by casting said cast steel, which suppresses various defects in a cast state and has internal quality equal to a forged steel product and excellent machinability.

[0015] The present inventors have been tried to develop a cast steel and a mold, which can satisfy the following targets at the same time to solve the above-mentioned problems. The targets are:

[0016] 1) To prepare a mold material by casting to significantly reduce cutting for finishing a mold, whereby working time and working costs are significantly reduced;

[0017] 2) To develop a cast steel from which a mold having sufficient hardness is obtained by casting; and

[0018] 3) To realize an internal quality of a cast steel, which is substantially equal to that of a forged steel, in composition series containing high S, and as cast, to simultaneously solve machinability and casting defects, which are problems in a cast steel mold.

[0019] Here, the term “internal quality” means not only the internal quality of cast products but also includes the quality of surface portions newly formed by cutting.

[0020] It is noted that if the above-mentioned problems are solved, the above-described cast steel can be of course applied to a cast steel for general machine's parts and the like.

[0021] As the result of the above-mentioned development, the following knowledge could be obtained.

[0022] a) When a large amount of S is contained in steel for the purpose of improvement of machinability and a required amount of Ti and Zr is added, sulfide containing Ti and Zr is produced so that no MnS is included at all or the production of MnS is remarkably suppressed. As the result, sulfide containing the Ti and Zr is finely dispersed in a cast steel.

[0023] b) As the result of the above a), machinability is not only improved, but also a solidification form different from a conventional case can be obtained. Even if the cast steel has high S content, the segregation of S is significantly improved as cast and at the same time columnar crystal in the steel is reduced or disappeared. Thus, not only defects such as porosity and segregation, but also projections and depressions of cutting-machined surface due to coarse columnar crystal is improved, whereby epoch-making internal quality equal to that of forged steel can be obtained.

[0024] The gist of the present invention completed based on the above-mentioned knowledge is as follows.

[0025] I) A cast steel having excellent internal quality and machinability after casting, characterized by comprising: by mass %, C: 0.02-0.45%, Si: 0.1-2.5%, Mn: 0.1-2.5%, P: 0.10% or less, S: 0.02-0.60%, N: 0.020% or less, and Al: 0.001-0.03% and further comprising one or two of Ti: 0.05-0.25% and Zr: 0.05-0.50%, and the remnants Fe and impurities, and also characterized in that effective Ti equivalent weight (Ti *) given by the following expression (1) satisfies a relation given by the following expression (2) and said cast steel contains sulfide containing one or more of Ti and Zr, and which is used as cast.

Ti*=Ti+0.53×Zr−3.4×N  (1)

Ti */S≧1.5  (2)

[0026] II) In the cast steel described in said I), in place of a part of Fe, one or two or more elements selected from groups consisting of, by mass %, Cr: 0.2-9.0%, Ni: 0.2-2.0%, Mo: 0.05-2.0% and V: 0.01-1.5% may be contained.

[0027] III) In the cast steel described in said I) or II), in place of a part of Fe, one or two elements selected from groups consisting of, by mass %, Cu: 0.1-3.0% and 0 (oxygen): 0.005-0.020% may be contained.

[0028] IV) A mold produced by casting the cast steel described in any one of said I) to III).

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] A preferred embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0030]FIG. 1 is a view showing a photograph of a macro-structure of an ingot in a cast steel containing none of Ti and Zr;

[0031]FIG. 2 is a view showing a photograph of a macro-structure of an ingot in a cast steel containing Zr;

[0032]FIG. 3 is a view showing a photograph of a micro-structure of inclusions in the center of an ingot in a cast steel containing Ti; and

[0033]FIG. 4 is a view showing a photograph of a micro-structure of inclusions in the center of an ingot in a cast steel containing Zr.

DETAILRED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] The following description is merely exemplary in nature and is in no way intended to limit the invention or its application or uses.

[0035] A structure and chemical composition of a cast steel according to the present invention will be described below in detail. It is noted that the “%” in the content of each element means “mass %” if not stated in the following descriptions.

[0036]FIG. 1 is a view showing a photograph of a macro-structure of an ingot in a cast steel containing none of Ti and Zr. A 143 mm-diameter ingot was produced by the use of a cast steel having a steel number of 16 in an example described later, and a photograph of its macro-structure was taken. Although a crack or an extreme macro-segregation were not recognized in the center of the ingot, the structure of the ingot is coarse and a range of about 30% in a section of the ingot from an outer surface of the ingot to a radial direction forms a columnar crystal structure.

[0037]FIG. 2 is a view showing a photograph of a macro-structure of an ingot in a cast steel containing Zr. A 143 mm-diameter ingot was produced by the use of a cast steel having a steel number of 2 in an example described later, and a photograph of its macro-structure was taken. The structure of the ingot was very fine and columnar crystal structure was not recognized. Thus the structure of this case is equal to the macro-structure of a hot-forged steel.

[0038]FIG. 3 is a view showing a photograph of a micro-structure of inclusions in the center of an ingot in a cast steel containing Ti (a steel number of 1 in examples), and FIG. 4 is a view showing a photograph of a micro-structure of inclusions in the center of an ingot in a cast steel containing Zr (a steel number of 2 in examples).

[0039] From the result of composition analysis of sulfide obtained by a point analysis in EPMA of inclusions, it has been found that the inclusions in FIG. 3 are inclusions most parts of which contains Ti sulfide, and the inclusions in FIG. 4 are inclusions most parts of which contains Zr sulfide.

[0040] This phenomenon is controlled by Ti, Zr, S and N in element components in cast steel. That is, when effective Ti equivalent weight given by the above-mentioned expression (1) by the use of these element content satisfies the above-mentioned expression (2), first at the time of solidification, a small amount of Ti or Zr reacts with N to form its nitride, and the remaining most parts of Ti or Zr produce sulfide. Since this sulfide accelerates solidification in one go without involving a large concentration of S, P and the like at a finally solidified portion where a solidified form is changed, the segregation of elements is improved and at the same time casting defects such as porosity and the like associated with solidification shrinkage are improved.

[0041] Said expression (1) expresses the amounts of Ti and Zr, which are reactable as sulfide. When a relation given by the expression (2) is satisfied, all S becomes sulfide of Ti or Zr stoichiometrically. In fact, after solidification, a small amount of MnS is sometimes observed. However the amount of MnS is an extent of no problem. When a value of the left side of the expression (2) becomes less than 1.5, a large amount of MnS is observed and no improvement of a solidified structure and the associated defects is recognized.

[0042] It is noted that when Ti content or Zr content is too much with respect to S content, an amount of production of Ti carbide or Zr carbide is increased thereby to deteriorate ductility or machinability. Therefore, it is preferred that the value of the left side of the expression (2) is 10 or less.

[0043] The reasons why ranges of contents of the respective elements defined in the present invention were defined as mentioned above will be described below.

[0044] C:

[0045] C (Carbon) is an effective element for enhancing the strength of cast steel at low cost. Since machinability is important in plastic molds and the like, it is necessary to reduce C content as low as possible to lower the hardness of steel material, so that tool wear is reduced. However, when the C content in cast steel is less than 0.02%, a decrease in hardness of the steel is remarkable, and stripping of the surface of the steel is generated during cutting-machining. Thus on the contrary machinability deteriorates. On the other hand, when C content exceeds 0.6%, hardness of the cast steel reaches too high, machinability and ductility are deteriorated. Accordingly, the range of C content was defined to 0.02-0.45%. The preferable range of C content is 0.08-0.45%.

[0046] Si:

[0047] Si (Silicon) is an effective element for deoxidation and improving machinability. When Si content is less than 0.1%, the effects are not obtained sufficiently. Further, a flow of molten steel during casting is blocked, whereby the filling properties of molten steel in a mold are insufficient. On the other hand, Si content exceeds 2.5%, which is very high, an effect of improving machinability and ductility are lowered. Accordingly, the range of Si content was defined to 0.1-2.5%. The preferable range of Si content is 0.5-2.0%.

[0048] Mn:

[0049] Mn (Manganese) is an element effective as an deoxidizer. However, when a large amount of Mn is contained, machinability is deteriorated. When Mn content is less than 0.1%, an deoxidation effect is not obtained. On the other hand, when Mn content exceeds 2.5%, machinability is deteriorated. Accordingly, the range of Mn content was defined to 0.1-2.5%. The preferable range of Mn content is 0.1-1.5%.

[0050] P:

[0051] P (Phosphorus) promotes segregation of P during solidification and ductility is deteriorated. On the other hand, P acts on improving the machinability. P may be contained as impurities depending on an aim of use of cast steel, and may be positively contained in the steel. For example, when ductility is taken very seriously, P content is preferably 0.02% or less. When the machinability is taken very seriously it is preferred that P content of 0.05% or more may be contained. However, when the P content exceeds 0.10%, deterioration of toughness becomes remarkable, and P content was defined as 0.10% or less.

[0052] S:

[0053] S (Sulfur) is an element contained for improving machinability. When S content is less than 0.02%, the generation of Ti sulfide or Zr sulfide is insufficient and a sufficient effect of improving machinability cannot be obtained. On the other hand, when S content exceeds 0.60%, large amount of FeS and MnS are generated, resulting in accelerating segregation, voids and cracking during solidification. Accordingly, the range of S content was defined to 0.02-0.60%. A preferable range is 0.05-0.40%, and a more preferable range is 0.05-0.20%.

[0054] Ti and Zr:

[0055] Ti and Zr are elements each having an effect of generating sulfide. In the present invention any one or two of these elements be contained in cast steel. In each content of less than 0.05%, sufficient sulfide cannot be generated. Accordingly, a large amount of MnS is generated whereby improvement of a solidified structure and improvement of defects associated with the structure are not found. On the other hand, when Ti exceeds 1.0% or when Zr exceeds 0.5%, an amount of generation of carbide other than sulfide is increased, thereby to deteriorate machinability and ductility.

[0056] Accordingly, the range of Ti content was defined to 0.05-0.25%. A preferable range of the Ti content is 0.10-0.25%.

[0057] Further, the range of Zr content was defined to 0.05-0.50%. A preferable range of the Zr content is 0.05-0.20%, and a more preferable range is 0.10-0.20%.

[0058] It is noted that these elements Ti and Zr must be contained so that an effective Ti equivalent weight given by the above-mentioned expression (1) satisfies the expression (2).

[0059] N:

[0060] N (Nitrogen) is an element to suppress the producing of Ti sulfide and Zr sulfide by producing nitride prior to generating Ti sulfide and Zr sulfide. Further, nitride generated is hard and breaks a tool tip thereby to reduce the tool life. Accordingly, the upper limit of N content was defined to 0.020%. The lower N content is the better. In N content of less than 0.002%, generation of nitride, which causes bad influence, due to N, can be preferably avoided.

[0061] Al:

[0062] Al (Aluminum) is a strong deoxidizing element and has an effect of improving ductility. In Al content of less than 0.001% the effects cannot be obtained. On the other hand, when Al content exceeds 0.03%, a deoxidizing effect is saturated and machinability is decreased. Accordingly, the range of Al content was defined to 0.001-0.03%.

[0063] It should be noted that Al content in the present invention means acid-soluble Al content (sol. Al content).

[0064] Cr:

[0065] Cr (Chromium) is an element having effects of enhancing strength, ductility and heat resistance in steel. Cr is not necessarily contained, but if improvement of strength, ductility and heat resistance are required, Cr content of 0.2% or more can obtain these effects. Particularly, in a hot-forging mold, Cr content of 3.0% or more is preferred. On the other hand, when Cr content exceeds 9.0%, ductility is deteriorated. Accordingly, the range of Cr content was defined to 0.2-9.0%. A preferable Cr content is 0.5-5.0%.

[0066] Mo:

[0067] Mo (Molybdenum) is an element having effects of improving strength, ductility and high-temperature strength in steel. Mo is not necessarily contained, but if improvement of strength, ductility and high-temperature strength are required, Mo content of 0.05% or more can obtain these effects. On the other hand, even if Mo content exceeds 2.0%, the effect of improved high-temperature strength is saturated, and ductility is deteriorated. Accordingly, the range of Mo content was defined to 0.05-2.0%. A preferable Mo content is 0.05-1.0%.

[0068] Ni:

[0069] Ni (Nickel) is an element having an effect of improving strength and ductility. Ni is not necessarily contained, but if improvement of ductility is required, Ni content of 0.2% or more can obtain the effect. On the other hand, when Ni content exceeds 2.0%, the effect of improved ductility is obtained. However, since a Ni element is expensive, the economical efficiency is lost. Accordingly, the range of Ni content was defined to 0.2-2.0%. A preferable range of Ni content is 0.2-1.5%.

[0070] V:

[0071] V (Vanadium) is an element having an effect of improving strength and having small ductility deterioration by allowing V to contain in steel. V is not necessarily contained, but if improvement of strength is required, V content of 0.01% or more can obtain the effect. On the other hand, when V content exceeds 1.5%, the ductility is deteriorated. Accordingly, the range of V content was defined to 0.01-1.5%. A preferable range of V content is 0.05-0.5%.

[0072] Cu:

[0073] Cu (Copper) is an element having effects of improving machinability, strength and ductility in steel. Cu is not necessarily contained, but if improvement of machinability, strength and ductility are required, Cu content of 0.1% or more can obtain these effects. On the other hand, when Cu content exceeds 3.0%, the ductility is deteriorated. Accordingly, the range of Cu content was defined to 0.1-3.0%. Apreferable range of Cu content is 0.2-1.0%.

[0074] O:

[0075] O (Oxygen) forms oxides of low-melting points by suitably allowing oxygen to contain in steel to have an effect of improving machinability. O is not necessarily contained, but if improvement of machinability due to the formation of oxides is required, O content of 0.005% or more can obtain the effect. On the other hand, when O content exceeds 0.020%, huge inclusions are formed in steel and finishing accuracy of mold surface is decreased and the ductility is deteriorated. Accordingly, the range of O content was defined to 0.005-0.020%. A preferable range of O content is 0.010-0.020%.

[0076] Here, O (Oxygen) means all oxygen in steel.

[0077] Heat Treatment:

[0078] In an ingot obtained by the present invention the microstructure, hardness and ductility can be improved by the subsequent heat treatment. Further, to improve the machinability the ingot can be softened to a required hardness. This heat treatment can be carried out by steps of heating the ingot to 850-1050° C., normalizing or hardening the heated ingot, and performing a softening treatment of the ingot by tempering at 700° C. or less or annealing the ingot or tempering and annealing the ingot.

EXAMPLES

[0079] Test steels, having chemical compositions shown in Table 1 on the next page, were melted and prepared cylindrical ingots each having a diameter of 143 mm.

[0080] After performing macro-tests of the ingots as cast, the following tests were carried out.

[0081] [Hardness] By use of test plate used in the macro-test, hardness of the center position, a half position of the radius and a position near the surface of each of said ingots was measured in accordance with a Brinell Hardness Testing Method defined in JIS Z 2243, and evaluated the respective ingots by average value of the measured hardness.

[0082] [Ductility] In accordance with a Charpy Impact Testing Method defined in JIS Z 2242, U-notch test pieces (full size test pieces each having a width of 10 mm defined in JIS Z 2202) were taken from positions on ½ of a radius, which are in directions parallel to the center axis of the cylindrical ingot, and impact values of the respective test pieces were measured at a room temperature.

[0083] [Milling Workability]

[0084] After taking the respective test pieces for hardness tests and impact tests, the respective ingots were cut in a cylindrical axial direction of the ingot, milling TABLE 1 Steel Chemical Composition (Mass %, Remnants Fe and Impurities) No. C Si Mn P S Ti Zr N Al O Others 1 0.10 1.01 0.12 0.010 0.052 0.49 — 0.0009 0.018 — — 2 0.10 1.06 0.23 0.020 0.055 — 0.43 0.0038 0.010 — — 3 0.08 0.48 0.21 0.012 0.340 0.68 — 0.0043 — — Cr: 0.56, Mo: 0.23 4 0.07 0.23 0.75 0.063 0.0580 0.86 0.48 0.0051 — — 5 0.09 0.47 0.21 0.014 0.180 0.08 0.44 0.0035 0.015 — Cu: 1.18 6 0.25 0.67 0.22 0.038 0.040 0.26 — 0.0068 0.002 0.0098 Cu: 2.45 Cu: 0.16, 7 0.23 0.48 0.67 0.025 0.080 0.18 — 0.0098 0.012 0.0087 Cr: 1.53 8 0.24 0.16 1.20 0.013 0.058 0.12 — 0.0089 0.001 0.0150 Cr: 0.12 9 0.20 0.12 1.77 0.022 0.076 0.23 0.05 0.0055 0.028 0.0057 Ni: 1.02 10 0.08 1.51 0.78 0.026 0.093 0.24 — 0.0120 0.006 — Ni: 1.68 11 0.22 1.67 0.28 0.086 0.150 — 0.49 0.0069 0.009 — V: 0.19 12 0.38 1.22 0.45 0.059 0.059 0.15 — 0.0079 0.009 0.0058 V: 0.14 13 0.26 1.02 0.80 0.009 0.021 0.05 0.06 0.0059 0.005 — Cr: 1.48, V: 0.10 14 0.56 0.23 0.89 0.035 0.034 0.12 — 0.0144 0.003 — — 15 0.35 0.76 0.58 0.007 0.026 0.18 — 0.0036 0.002 0.0052 Cr: 6.8, V: 0.82, Mo: 1.57 16 0.08 0.23 0.23 0.022 0.060 0.04* 0.03* 0.0057 0.002 0.0052 — 17 0.07 0.58 2.64* 0.022 0.160 0.05 — 0.0029 0.043* — — 18 0.10 0.27 0.34 0.120* 0.050 0.27 0.15 0.0236* 0.001 0.0068 Cr: 1.05 19 0.29 0.75 1.18 0.027 0.025 1.09* —* 0.0068 0.017 — — 20 0.24 0.25 0.82 0.035 0.016* —* —* 0.0086 0.020 — Cr: 1.69, V: 0.13 21 0.64* 0.25 0.96 0.029 0.014* —* —* 0.0096 0.001 — — 22 0.42 0.22 0.92 0.025 0.540 —* —* 0.0080 0.024 — Cr: 1.2

[0085] TABLE 2 Remarks (Heat Value of Value of Impact Macro- Milling Treatment Steel Expression Expression Hardness Value Structure Workability and No. (1) (2) (HB) (J/cm²) (*1) (*2) the like) 1   0.49 9.4 153 109 ∘ 0.12 2   0.21 3.9 159 107 ∘ 0.13 3   0.67 2.0 155 122 ∘ 0.06 4   1.10 1.9 142 109 ∘ 0.04 5   0.30 1.7 192 137 ∘ 0.13 6   0.24 6.8 248 85 ∘ 0.20 7   0.15 1.8 210 96 Δ 0.22 8   0.09 1.5 186 76 Δ 0.12 9   0.24 3.1 171 131 ∘ 0.22 10   0.20 2.1 185 98 ∘ 0.10 11   0.24 1.6 214 129 ∘ 0.21 12   0.12 2.1 206 108 ∘ 0.09 13   0.06 2.9 223 63 ∘ 0.23 14   0.08 2.4 267 41 Δ 0.24 After normalizing at 950° C., tempered at 600° C. 15   0.17 6.5 312 23 Δ 0.27 After normalizing at 1020° C., tempered at 700° C. 16   0.04 0.6* 147 101 x 0.35 17   0.04 0.3* 177 15 xx 0.46 18   0.27 5.4 155 28 x 0.58 19   1.07 42.7 272 5 xx >0.60 The tool material was broken by cutting 2000 mm of the steel. 20 −0.03 −1.8* 206 35 x 0.43 21 −0.03 −2.3* 220 32 xx 0.56 Tempered at 550° C. 22 −0.03 −0.1* 263 15 x 0.45 Tempered at 600° C.

[0086] Table 2 on the last page shows the values of expressions (1) and (2), the results of said tests and evaluation results of the macro-structures.

[0087] The steels of steel Nos. 1-15 in the test steels shown in Table 1 are the steels according to the present invention. Their chemical compositions of the steels each satisfy a range of the chemical compositions defined in the present invention. Further, as shown in Table 2, the values of expression (1), which are computed based on the chemical compositions of steels, satisfy the relation given by expression (2).

[0088] The macro-structures of test steels were evaluated as follows.

[0089] The macro-structure of a steel of steel No. 16 (Comparative Steel) shown in FIG. 1 was expressed by a mark “X” because it is an undesirably coarse structure, the macro-structure of a steel of steel No. 2 (A steel of the present invention) shown in FIG. 2 was expressed by a mark “∘” because it is a fine and excellent structure, macro-structures, which are the same as and more excellent than the No.2 steel, are also expressed by the mark “∘”. Further, the macro-structures of test steels having structure levels between the marks “X” and “∘” were expressed by a mark “Δ”.

[0090] The macro-structures of the steels of the present invention are substantially evaluated as “∘”. Some test steels (Steel Nos. 7, 8, 14 and 15) of the present invention were evaluated as “Δ”, but this is because S content was comparatively small and an amount of generation of Ti sulfide or Zr sulfide, which contributes to improvement of solidified structures, was small.

[0091] Since the test steels of steel Nos. 14 and 15 have high hardness as cast, they are normalized and annealed and subsequently softened so that cutting working after casting was made easy. Particularly, since the test steel of steel No. 15 is used for hot-forging mold, it is cut or machined and subjected to heat treatment of hardening and tempering.

[0092] Impact values of all test steels of the present invention are good except for the steel of steal No. 15, in spite of the fact that S content is in high level.

[0093] Further all steels of the present invention each have small amount of tool wear in its milling working, and have excellent machinability.

[0094] The steel of steel No. 7 containing Cr and Cu, and the steel of steel No. 13 containing Cr and V increased hardness but had improved ductility.

[0095] It has been found that the steels of steel Nos. 9 and 10 containing Ni each have an effect of improved ductility. Further, it has also been found that the steel of steel No. 3 containing Cr and Mo each have effects of improved strength and ductility.

[0096] The steels of steel Nos. 11 and 12 containing V have been improved in strength and ductility. The steel of steel No. 5 containing Cu has been improved in machinability in spite of increased strength of the steel.

[0097] The steel of steel No. 6 containing Cu and O (oxygen) has been improved in machinability. The steels of steel Nos. 7 and 8 containing O (oxygen) each have been improved in machinability.

[0098] On the other hand, the steels of steel Nos. 16-22 are comparative steels, and at least one of steel components or relations given by the expression (2) is out of the ranges defined in the present invention.

[0099] The steels of steel Nos. 16, 17, 20, 21 and 22 among said steels do not satisfy the relation given by the expression (2), and the evaluation of the macro-structures thereof are X, or XX. Further, the steels of steel Nos. 18 and 19 each satisfy the relation given by the expression (2). However, since in the respective steels, P content and Ti content are excessive, segregation of P or Ti is significant, and remarkable segregation, which leads to a crack at the center of the steel ingot, was observed.

[0100] The steel of steel No. 16 had a macro-structure including remarkably grown columnar crystals. As a result a remarkable P or S segregation structure has been found at the center of the steel ingot. Further, even a porosity and a crack were observed at a position where the segregation structure exists. Also the steel of steel No. 20 is the same as the steel of steel No. 16.

[0101] Further, the steel of steel No. 22 had a lower impact value, and had poor ductility.

[0102] Since the steel of steel No. 17 had too high Mn and Al contents, and the steel of steel No. 18 has excessively high N content, they had poor milling workability. Since the steel of steel No. 19 had excessively high Ti content, a large amount of TiC was generated. Further, since it has high hardness it had poor milling workability. Further, since the steel of steel No. 21 had excessively high C content and poor S content, it had poor milling workability.

[0103] As apparent from the above-described results, by controlling contents of Ti, Zr and S in a steels to a suitable range, fine Ti and Zr sulfides are generated so that a cast steel having excellent internal quality and machinability after casting and a mold having excellent machinability, whose internal quality as cast is comparable to that of a forged steel product, can be produced.

[0104] According to the cast steel and the casting mold according to the present invention, by limiting steel compositions with respect to special components and controlling contents of Ti, Zr and S in a steel to a suitable range, fine Ti and Zr sulfides are generated so that a cast steel having excellent internal quality and machinability after casting can be ensured. Then, by casting this cast steel a mold having excellent machinability, whose internal quality as cast is comparable to that of a forged steel product, can be produced. Therefore, the mold of the present invention can be widely applied to use, which severely requires excellent surface properties in products for deep-engraving in which a worked surface reaches the internal portion of the section material or for finishing.

[0105] Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A cast steel having excellent internal quality and machinability after casting, characterized by comprising: by mass %, C: 0.02-0.45%, Si: 0.1-2.5%, Mn: 0.1-2.5%, P: 0.10% or less, S: 0.02-0.60%, N: 0.020% or less, and Al: 0.001-0.03% and further comprising one or two of Ti: 0.05-0.25% and Zr: 0.05-0.50, and the remnants Fe and impurities, and also characterized in that effective Ti equivalent weight (Ti *) given by the following expression (1) satisfies a relation given by the following expression (2) and said cast steel contains sulfide containing one or more of Ti and Zr, and which is used as cast. Ti*=Ti+0.53×Zr−3.4×N  (1) Ti*/S≧1.5  (2)
 2. A cast steel having excellent internal quality and machinability after casting, according to claim 1, characterized in that in place of a part of Fe, one or two or more elements selected from groups consisting of, by mass %, Cr: 0.2-9.0%, Ni: 0.2-2.0%, Mo: 0.05-2.0% and V: 0.01-1.5% be contained.
 3. A cast steel having excellent internal quality and machinability after casting, according to claim 1, characterized in that in place of a part of Fe, one or two elements selected from groups consisting of, by mass %, Cu: 0.1-3.0% and 0 (oxygen): 0.005-0.020% be contained.
 4. A cast steel having excellent internal quality and machinability after casting, according to claim 1, characterized in that in place of a part of Fe, one or two or more elements selected from groups consisting of, by mass %, Cr: 0.2-9.0%, Ni: 0.2-2.0%, Mo: 0.05-2.0% and V: 0.01-1.5% be contained, and one or two elements selected from groups consisting of, by mass %, Cu: 0.1-3.0% and 0 (oxygen): 0.005-0.020% be contained.
 5. A mold produced by casting said cast steel according to claim
 1. 6. A mold produced by casting said cast steel according to claim
 2. 7. A mold produced by casting said cast steel according to claim
 3. 8. A mold produced by casting said cast steel according to claim
 4. 